Comparative Study of Dynamics and X-ray Emission of Several Plasma Focus Devices

Shan, Bing (2000) Comparative Study of Dynamics and X-ray Emission of Several Plasma Focus Devices. PhD thesis, Nanyang Technological University, Singapore.

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Abstract

This thesis describes the theoretical and experimental investigations of the x-ray radiation properties and the plasma dynamics in several plasma focus devices. The purpose of the research is to optimize the x-ray radiation behavior of the plasma focus and to seek the conditions for generating shorter wavelength x-rays with argon gas. The characteristics of neon and argon plasmas are investigated in detail based on the corona model. The results show that much higher temperature and energy are required to generate x-ray radiative argon plasma. This explains why it is much easier to obtain soft x-rays from the neon plasma focus. An improved plasma focus model is proposed with emphasis on detailed analysis of energy transfer processes and thermodynamic processes. Agreement between the experiment results and the computations for neon and argon is noted. The model is extended to describe, for comparison purpose, the gas-puff plasma focus. Modifications are made based on the measured gas pressure profile and assumed gas composition. Applying the modified model to the gas-puff focus of Gunma University reveals that the gas-puff scheme is more efficient to heat the plasma and to generate x-rays. And it also results in a more stable plasma column. By comparing with published results, agreement is obtained in the major points regarding plasma dynamics, plasma column stability and appearance, plasma temperature, and x-ray radiation properties. Experiments are mainly carried out with the repetitive NX2 plasma focus, under various charging voltage and gas pressure with a series of anode lengths. A Rogowski current derivative coil, a resistive voltage divider, and a PCD x-ray detector are installed iii to measure the current derivative, tube voltage and x-ray signal. All these detectors have been carefully calibrated to give quantitative information. A fast four-channel data acquisition system has been set up to record the fast data flow from the high-repetitive rate machine. X-ray radiation properties from the plasma focus with neon have been investigated in detail. The optimized working region is located by scanning the working conditions. The x-ray yield in this region is ~ 12 Joule/shot with the peak power in the GW order. The correlation between x-ray yield with various operating parameters such as repetition rate and reproducibility is studied. The dynamics of the plasma focus is studied with the recorded current and voltage signal. The correlations of discharge current, focusing time, tube inductance, axial speed and detail features of x-ray radiation are investigated. Efforts of generating argon x-rays in NX2 are made under various experimental conditions. However, in argon the NX2 device does not focus in the expected high voltage, low pressure working region. The experiments are then carried out in the UNU/ICTP PFF (United Nations University/International Center for Theoretical Physics Plasma Focus Facility) to study the mechanism for argon x-rays. The characteristic Ar lines are recognized from the measured signal. The neon experiments are also carried out with this machine for comparison. The results show the optimized working pressure for neon x-ray is much higher than for argon. This confirms the theoretical conclusions that much higher energy and temperature are required to generate x-ray from argon plasma.

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